Thermally activated heat pumps based on the absorption principle hold great promise for meeting the combined environmental goals of higher energy efficiency (reduced CO.sub.2 emissions) and zero ozone depletion for space conditioning applications. However, the joint achievement of high efficiency, simplicity, and low cost has heretofore proved to be elusive.
The use of solid sorbents in single-effect intermittent cycle heat pumps or refrigerators is well known. Beyond the generic advantages of all sorption cycles (no compressor, actuated by heat vice shaft power, and no use of CFCs), solid sorbent use presents the added advantages that no sorbent or refrigerant pumps or valves are required (in certain configurations) and the sorbent is reasonably well localized. There are, however, many disadvantages: high latent heat of sorption causes very low coefficient of performance (COP); achieving continuous heat flow requires multiple units connected via complex valving arrangements; the heat release rate tends to be highly uneven, and that coupled with the periodic requirement to change between absorb and desorb results in substantial idle or lightly loaded periods. To compensate for the light load periods, the apparatus must be highly loaded the remainder of the time, and the highly loaded periods determine the heat exchange surface requirements. An additional disadvantage for solid absorbents (chemisorbents) is their monovariant equilibrium, i.e., pressure is solely a function of temperature, and not of refrigerant (sorbate) content. Thus each solid absorbent operates at a unique lift (relative to the pure refrigerant), and if the lift requirement changes, e.g., due to varying ambient temperature, the sorbent cannot adjust.
Yet another problem with historical chemisorbents was that the characteristic extreme sorbent volume changes (shrinking and swelling) caused the sorbent bed to compact and deactivate. That problem was largely overcome by either of two techniques. In the first technique, the chemisorbent was combined with additions of either viscous liquid (LiNO.sub.3) and/or of various inert conductive media, especially intricate porous structure such as activated carbon or exfoliated graphite. Prior art disclosures of this technique are found in U.S. Pat. Nos. 2,986,525 and 4,595,774. In the second technique, the salt is prevented from swelling to its full extent, and is kept compressed within a volume smaller than that which it tends to occupy. The resulting pressure exerted on the salt by the container causes tile salt to take up a fixed position which is not shifted during desorption. This ensures more rapid reaction and good heat transfer. The compressed condition of the salt is achieved by partially loading the container with granular desorbed salt and then absorbing in situ until the reactor is full of solid, and then absorbing a minor additional amount. This technique is disclosed in U.S. Pat. Nos, 2,326,130 and 2,384,460.
For direct-fired space-conditioning applications, the most severe limitation of single-effect solid sorbent intermittent cycles is the low COP. As a result, various multi-effect cycles have been proposed. Unfortunately, they have also increased complexity, by any of several mechanisms: a) sorbate valves and/or throttles; b) sorbent-to-sorbent heat exchange through heat transfer loops comprised of two heat exchange surfaces and pump(s) and/or valves; c) complex heat transfer loop valving; d) excessive generator temperature requiring use of hot oils having poor heat transfer characteristics; and e) multiple sorbent beds are interconnected in conjunction with more sorbate than one sorbent bed can hold, which risks liquefying one of the sorbent beds at shutdown or abnormal conditions (all the sorbate migrates to the highest affinity sorbent).
Examples of disclosures of multi-effect solid sorbent heat pumps and their attendant complexities from the above list are: U.S. Pat. No. 5,083,607 (be); U.S. Pat. No. 5,057,132 (abe); U.S. Pat. No. 5,079,928 (abcde); U.S. Pat. No. 5,025,635 (abcde); U.S. Pat. No. 5,174,367 (bc); and U.S. Pat. No. 5,046,319 (abc). Multiple effect heat pumps based on hydrides also experience the above difficulties, e.g., U.S. Pat. Nos. 4,623,018 and 5,174,367.
Rotary sorption heat pumps have been proposed. By arranging a multiplicity of single-effect intermittent cycle sorption zones or modules on a rotating frame, it is possible to achieve continuous heat pumping without either sorbent valves or heat transfer valves. Examples are disclosed in U.S. Pat. Nos. 4,169,362, 4,478,057, 4,574,874, 4,660,629, and 5,157,937.
Thermosyphons (also known as gravitational heat pipes) are frequently used to supply heat to or remove heat from intermittent sorption cycles. The thermosyphon may be hermetically separate from the sorption cycle, as in U.S. Pat. Nos. 2,544,916, 2,596,523 and 4,993,234. Alternatively the thermosyphon can be integrated directly into the sorption cycle, using a common working fluid and a common component, as in U.S. Pat. Nos. 2,446,636, 2,452,635, and 4,744,224. Also known are three-way thermosyphons, wherein one component is alternately a sink receiving heat from an external supply and then a source delivering heat to a different external sink, as in U.S. Pat. No. 2,293,556. Recently two new uses of thermosyphons in intermittent sorption cycles have been disclosed. In U.S. Pat. No. 5,083,607, a valved stationary thermosyphon transfers latent heat from one chemisorbent to another during one step of a two-step cycle, making possible a double-effect cycle. In U.S. Pat. No. 5,157,937, valveless rotating thermosyphons exchange sensible heat between two beds of the same adsorbent (also known as physisorbent), thus reducing changeover sensible heat losses and increasing COP. The two ends of the thermosyphon are effectively mirror images of each other, and each does the same two things. The rotation causes the thermosyphon to reverse direction of heat flow, i.e., heat flow is always from whichever end is lower to the other higher end. Both of the above disclosed thermosyphons are hermetically isolated from the sorption cycle.
What is needed, and included among the objects of this invention, are apparatus and corresponding process for at least one of heat pumping, refrigeration, and space conditioning, which apparatus achieves the simplicity and constant duty of rotary cycles, plus preferably also the high COP of multi-effect cycles, without the complications of refrigerant valves or seals, or of heat transfer liquid pumps, valves, or seals. It will preferably have a capability of being directly fired, while positively avoiding combustion gas contamination of heating air, and with no requirement for pumped circulation of hot oil. It will preferably achieve direct heating and/or cooling of conditioned space air with no intervening heat transfer liquid loop, and with minimal refrigerant distributed in multiple hermetically separate modules whereby the maximum refrigerant leakage possible from any single leak is essentially insignificant.
The apparatus will, in the multiple-effect embodiments, preferably transfer internal heat valvelessly from module to module, using rotation to appropriately turn the thermosyphon on and off. The newly disclosed multi-effect embodiments will preferably be simpler and more efficient than existing multi-effect embodiments.
Another objective is to disclose advantageous new multi-effect configurations which can be applied in stationary embodiments in addition to or in lieu of rotary embodiments.